1. Field of the Invention
The present invention relates to a solar cell and a method of manufacturing a solar cell, and more particularly to a solar cell having a textured structure on the surface of a semiconductor crystal substrate and a method of forming such a textured structure.
2. Description of the Related Art
Solar cells are generally made of a semiconductor material such as silicon or the like, and have a flat mirror surface for receiving sunlight. The flat mirror surface is coated with an anti-reflection film such as a silicon nitride film or the like for preventing the applied sunlight from being reflected from the flat mirror surface for an increased efficiency to trap sunlight. Another approach to trap more sunlight for higher efficiency is to provide a textured structure on the surface of the semiconductor substrate of the solar cell.
The textured structure comprises minute surface irregularities provided on the surface of the semiconductor substrate. The textured structure can be formed by dipping a substrate surface whose crystalline surface is represented by the (100) surface in a special etching liquid such as of an acid or alkaline. Specifically, when the (100) surface is etched by the etching liquid, it produces a pattern of pyramidal surface irregularities corresponding to a (111) surface. However, it is not easy to form the textured structure on the crystal substrate having (111) surface thereon, even though dipping in the special etching liquid. Further, the above term xe2x80x9ccrystalline surfacexe2x80x9d is also called xe2x80x9ccrystalline facexe2x80x9d or xe2x80x9ccrystalline planexe2x80x9d.
According to another process of producing a textured structure, a texture pattern is formed on a resist film on a substrate by photolithography, and then transferred to the surface of the substrate by etching. Other processes include a process of forming grooves mechanically in a substrate surface with a dicing saw and then etching the substrate surface, and a process of forming a textured structure on a substrate surface by laser. While these processes are applicable to monocrystalline or polycrystalline silicon substrates irrespectively of their crystalline surfaces, they involve complex steps and result in an increase in the cost of the textured structure. Another problem of these processes is that the textured structure thus formed causes mechanical damage to the substrate surface, tending to lower the performance of the resultant solar cell.
It is therefore an object of the present invention to provide a solar cell having a textured structure, which can be simply formed on the surface of a semiconductor crystal substrate independently of the crystalline structure thereof for trapping more sunlight applied thereto, and a method of manufacturing such a solar cell.
According to the present invention, there is provided a method of manufacturing a solar cell, comprising; bringing a front side of a semiconductor crystal substrate into contact with an electrolytic liquid containing a fluoride, placing an electrode in the electrolytic liquid, passing a current between the electrode and the semiconductor crystal substrate and applying light to the semiconductor crystal substrate to generate pairs of holes and electrons, and etching the semiconductor crystal substrate by combining the holes with ions in the front side of the semiconductor crystal substrate which is held in contact with the electrolytic liquid, thereby forming at least one surface irregularity structure.
The semiconductor crystal substrate comprises a monocrystalline silicon substrate or polycrystalline silicon substrate having a (111) surface.
Specifically, the semiconductor crystal substrate comprises a monocrystalline silicon substrate having a thickness up to 150 xcexcm.
According to the present invention, there is also provided a solar cell comprising a semiconductor crystal substrate having a thickness up to 150 xcexcm and a (111) surface, and a textured structure disposed on a surface of the semiconductor crystal substrate.
In the solar cell, the semiconductor crystal substrate comprises a monocrystalline silicon substrate or polycrystalline silicon substrate.
In the solar cell, the textured structure is formed by bringing the surface of the semiconductor crystal substrate into contact with an electrolytic liquid containing a fluoride, passing a current between an electrode placed in the electrolytic liquid and the semiconductor crystal substrate, and applying light to the semiconductor crystal substrate.
The above photo-electrolytic etching progresses when holes generated upon application of the light to the semiconductor crystal substrate and ions in the electrolytic liquid are combined with each other, thus easily forming a textured structure of surface irregularities on the front side of the semiconductor crystal substrate. The textured structure thus formed by the photo-electrolytic etching does not depend on the crystalline surface, the method and the solar cell are applicable to crystal substrates having the (111) surface. It is thus possible to manufacture inexpensively a solar cell having a textured structure for increasing the efficiency to trap sunlight, from a monocrystalline silicon substrate which provides high photoelectric conversion efficiency.
Since a textured structure of surface irregularities can be formed on the surface of a monocrystalline silicon substrate which has a small thickness of 150 xcexcm or less and a (111) surface, it is possible to provide a solar cell which has a high photoelectric conversion efficiency and which is flexible enough to allow itself to be applied to a curved surface.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.